This invention relates to an improved fluid ink, and more particularly to improved aqueous fluid ink compositions. Compositions prepared in accordance with preferred embodiments of the invention have lowered freezing points and are particularly well suited for use in printers, such as, an ink jet printer.
Many printers utilize fluid inks, such as an ink jet printer and a wire dot-type printer. In the ink jet printing process, printed dots are formed on a print medium by application of fine ink droplets in a desired pattern in accordance with a non-impact procedure. The wire-dot type ink printer is one wherein a fluid ink is placed on the tip of a wire and printing is performed in accordance with an impact procedure.
Since these types of printers utilize a fluid ink, they cannot function at temperatures lower than the freezing point of the fluid ink. For example, the printing operation cannot be utilized because the fluid ink freezes at low temperatures. Once freezing occurs, various problems arise in addition to the inability to print. The fluid ink is usually composed of several components, which when re-liquified after freezing, readily separate into these individual components. After re-liquification, it is necessary to remix the separated fluid ink by application of stirring or heat and stirring. In the case of an ink jet printer, the ink jet printing head may be damaged due to expansion of the fluid ink resulting from the freezing. In addition, freezing and re-liquification may cause formation or introduction of air bubbles into the fluid ink. This prevents sufficient pressure from forming in the pressure-application chamber for forming the fluid ink into the fine droplets required. Thus, it is impossible to perform printing of acceptable quality after the ink has frozen.
In order to avoid these problems, it is desirable to provide a fluid ink for use in printers which will not freeze at the temperatures at which the printer is operated. In addition to overcoming this freezing problem, it is necessary to maintain the ink at a low viscosity. Fluid inks for printers, particularly those for ink jet printers are aqueous inks having low viscosity and high surface tension which permit easily handling. These aqueous inks, however, have freezing points which are not much lower than that of water. For example, an aqueous ink having a viscosity of 2 centipoises, or less, at 20.degree. C. freezes at a temperature of -5.degree. C.
In view of these relatively high freezing points, many conventional aqueous inks, therefore, are not suitable for use at low temperatures. In order to prevent these inks from freezing by heating and/or insulating, for example, the printer must include a heat-insulation apparatus. This gives rise to the disadvantage that additional energy is required for such a heat source. Operation of a heater results in an increase in the amount of electricity to be consummed. Additionally, this additional element increases the production cost and size of the printer.
Oil based inks which are made up of organic solvents having low freezing points are also useful in the printers. These oil based inks; however, suffer from the disadvantage of small contact angles between most organic solvents and the recording medium. In view of this, the organic solvents penetrates unequally through the recording medium making it difficult to form fully satisfactory printing dots. Thus, the quality of the print letters formed using such an oil base ink is inferior to that of the letter formed from aqueous inks.
In general, aqueous inks suitable for use in an ink jet printer include principally water, a dye as the recording medium, and a humectant to prevent deposition of the dye due to evaporation of water. By increasing the amount of humectant, without addition of additional components, it is possible to lower the freezing point of the ink to much lower levels than that of water. For example, when glycerol is utilized as the humectant, a 20% by weight aqueous solution of glycerol has a freezing point of -5.degree. C., and a 40% by weight aqueous solution of glycerol has a freezing point of -15.degree. C. On the other hand, the viscosity of the 20% by weight solution is 1.8 centipoises, but the viscosity of the latter aqueous solution is increased to about 3.8 centipoises. Such increase in viscosity of the ink lengthens the time required for the meniscus of ink to return to the original level in an ink jet printer nozzle during formation of ink droplets. Therefore, it is not possible to increase the frequency of response of the recording head to a desired level. This leads to the disadvantage that the printing speed cannot be increased.
When attempting to lower the freezing point of a water-humectant-based printing ink, for example, containing no additional components and maintain the viscosity between the range of about 2 to 3 centipoises at 20.degree. C., it has not been possible to lower the freezing point to levels lower than about -15.degree. C. This is the case even by using a humectant capable of effectively lowering the freezing point, such as ethylene glycol. Accordingly, it would be desirable to provide a composition for a fluid ink wherein the freezing point could be lowered while maintaining the viscosity within the range of about 2 to 3 centipoises at 20.degree. C.